In the years to come, as the water crisis reaches the proportions of the energy crisis, we may begin to look at our water-hungry central sewage systems with the same angry eye we've recently turned toward fossil fuel burning power plants.

Luckily, technologies exist today which provide water-conservative waste disposal economically. In ver limited areas, they're beginning to be tested and tried.

Most, but not all, water-saving waste disposal systems are on-lot types. In a water-short area, these have the advantage of discharging the effluent (the liquid remaining after wastes have been removed) into the adjacent soil or air, just where it is most needed. A municipal plant, on the other hand, pumps the effluent away from the dry area, into a stream which flows on the next town and eventually to the ocean.

Though there are many types, most of the new on-lot plants attempt in some way to overcome the two major weaknesses of the conventional septic system: a dirty effluent, and unsuitability for heavy soils.

Aerobic tanks, which have been tested successfully in Kentucky, Maryland, and elsewhere, pump air into otherwise ordinary septic tanks to speed up decomposition of waste. The result is a much cleaner effluent than that going out of normal septic field. Another system dumps waste-eating bacteria into the collection tank, also with the objective of producting a cleaner end product.

Where soils are poor, evapo-transiration (ET) beds have been used to replace conventional drain fields. Instead of releasing the effluent into the ground, ET beds allow some of it to evaporate directly into the air, while the rest is transpired through the roots and leaves of greenery planted on top. The effect is a natural cleansing of the water as it passes through soil, plants and air.

Extensive research on ET beds is currently going on in a seven state project under the direction of the Virginia Polytechnic Institute. Where soils are porous enough, the ET bed can be combined with limited infiltration into the ground. Where it is very compact, soil can be mounded over the bed to create a more efficient surface for evaporation. The flexibility of the system may one day make it adaptable to a variety of conditions.

Aside from ecological benefits, individual systems also gain economic advantages over central sewers as water consumption drops.

Currently, according to Lee Paserew of the Montgomery County Planning Department, "the big problem in individual disposal is getting rid of it. The less you have, the less you have to get rid of."

This often means that the less water passing through an individual system, the less the system costs. Dr. Roy Ricci, president of Intex, Inc., which holds the patent for an aerobic system combined with an ETI mound, says that water conservation techniques could cut the cost of his product considerably.

"A large part of the expense," he says, "is in the mound. Recycling - lawn irrigation, for example - could cut the mound in half."

Conservation over the long run would reduce costs even more. Like most on-lot systems, the Intex design is engineered to carry 100 gallons of water per person per day. If water usage were cut, say to the 50 gallon per day average in Europe, says Ricci, a smaller tank could be used. Basically, the same criteria would apply to other systems.

Ultimately, this may mean that on-lot systems will become cheaper and more practical as water conservation becomes the norm, even as large central facilities begin to face problems brought on by lack of enough water for sewage transport. At that point, the only salvation for the large central systems will be some sort of retrofitting, perhaps to grinder and pressure pipes that are much smaller than the typical gravity-flow pipes currently used. But retrofitting a major sewer system is a multi-million dollar project. Looking seriously now at less expensive, ecologically sound on-lot systems makes more sense, before they are desperately needed.

Even the Environmental Protection Agency is coming to that conclusion. Not long ago Sen. Jennings Randolph (D-W.Va.) aruged heatedly that it is no longer economically feasible to build municipal sewers in small rural communities. Costs can skyrocket to as much as $10,000 per house.

Since then, EPA has initiated a funding program to allow communities to try sewerage alternatives, provided they are used on a system basis (a number of dwellings, not just one), with all the dwellings pat of a public sanitary district. One such experimental district is being tried in Montgomery County.

The approach makes sense. Always before, rural sanitation needs were simply ignored. Only residents on the public sewer lines received and maintenance service at all. Septic systems on perfectly good soils - systems that may have worked indefinitely with proper care - failed because their unkeep was left to disinterested homeowners. The sanitation district assures that on-lot systems (most of them more sophisticated than normal septic tanks, and in need of more regular maintenance) get the service they required.

Unfortunately, the EPA program is small and still in its birth throes. Environmentalist Harold Leich, concerned that waste disposal is not being approached in the interests of either economy or ecology, suggested not long ago that EPA launch a nationwide effort "to insure that every promising sewerless device is tested on a large scale in actual use and carefully monitored." If no commercial device proved satisfactory, Leich proposed "a major federal research project" to find the answer to waterless waste disposal. This is not yet being done.

Perhaps it would be. As water conservation becomes more and more crucial, a genuine reassessment of priorities will be needed. Otherwise we may one day discover that we have spent billions on outdated sewers, and very little on alternatives that can dispose of waste in a water-hungry world.

Terry and Ellyn Bache are president and vice president of The Terhane Group, Inc., a Hagerstown-based building firm. Terry Bache is a member of the Washington County Planning Commission, which is collecting data on sewerage alternatives nationwide.